U.S. Pat. No. 4,751,814 discloses an air cycle thermodynamic conversion system including a gas turbine providing a flow of heated gases from the gas turbine exhaust; at least one air compressor for compressing ambient air; a heat exchanger including means for transferring heat from said flow of heated gas turbine exhaust gases to a compressed air from said air compressor to produce a heated compressed air; at least one air turbine connected to the heat exchanger responsive to said heated compressed air to drive said at least one compressor; said heated compressed air including an excess of energy beyond that required by said at least one air turbine to drive said at least one air compressor; and, means for delivering said excess of energy to a using process. By establishing the heated exhaust gas and the compressed air flow in the heat exchanger such that they both have about equal heat capacities, a minimum temperature gradient is maintained between them. The use of compressed air provides an air bottoming cycle.
The control of this known system requires control valves to regulate the flow rate of heated air, and its temperature, to the air turbine in order to regulate the power developed in the air turbine. This requires control valves of large dimensions and the control system is a slow working system. Thus no practicable control has been available up to-day. I have now discovered that a desired rapid and convenient control may be provided by establishing mutually rotary dependent torque axes of the gas turbine and the air turbine. This new arrangement makes it possible to control the system by only controlling the gas turbine, i.e. by controlling the fuel flow to the combustion chamber of the gas turbine. Control of fuel flow to the combustion chamber results in a rapid change of the gas turbine performance and of the total apparatus, due to the dependency of the torque axes of the gas turbine and the air turbine. This rapid control of power output from the gas turbine also compensates for the slow-reacting air bottoming cycle due to the large thermal inertia in the heat exchanger.